Development process and developer for producing hardened images upon exposed silver halide-colloid emulsions



Patented Aug. 16, 1938 UNlT ED STATES 2.126.899 DEVELOPMENT raoonss ANDDEVELOPER.

FOR PRODUCING HARDENED IMIAGES UPON EXPOSED SILVER HALIDE-COLLOIDEMULSIONS Ferdinand Leibcr, Berlin-Friedrichshagcn, Germany, assignor toNaamlooze Vennootschap: Nederlandsch Laboratorium de Spaarnestad,Haarlem, Netherlands, a corporation of the Netherlands I No Drawing.Application July 10, 1935, Serial No. 30,769. In Germany June 13, 1935 4Claims.

It is known that developing solutions comprising alkali andtrihydroxybenzene (pyrogallic acid), or some dihydroxybenzenes such ashydroquinone, chlorhydroquinone, pyrocatechine, cause 5 a hardening ofthe erriulsion colloid at the exposed and developed places if the usualaddition of sulphite to the developing solution is considerably reducedor entirely suppressed. This phenomenonmay be explained by the fact thatwith the development the substances contained in the developer give riseto the formation of a tannic acid as a product of oxidation which isadsorptively fixed by the emulsion colloid, which is of course formed inlarger proportion at those places where a more intensive reduction ofsilver halide to metallic silver occurs. The said developing solutionswhich are free from sulphite or poor in sulphite may therefore be usedto develop and harden images in an expo d emulsion layer which images inmany res cts correspond to those obtained by the exposure of chromatecolloids.

However, in actual practice the use of the hardening developers veryoften gives rise to difficulties such as on the one hand washing awaythe layer and on the other hand this layer becoming entirely insoluble,and the causes thereof could not be explained up to the present.Insolubility of the layer is for instance caused by oxidation of thedeveloping substance in the air during the development to tannic acid;the otherwise usual addition of sulphite in order to avoid a too fastoxidation cannot be used in the present case, for by addition of smallquantities the conservation is not sufficient and by addition of largerquantities no hardened image is formed. Those drawbacks form a baragainst the technical use of the hardening developers.

According to the invention the drawbacks of the washing away of thelayer and also the layer becoming insoluble are removed by thesimultaneous employment of trihydroxybenzene (pyrogalli c acid) and adihydroxybenzene, e. g. pyrocatechine in the same developer orsuccessively with the same developing process, and a hardened image isobtained which practically corresponds to the best hardened'chromategelatine images viz. they show a tanning exactly corresponding to thereacting quantities of light, or to the quantities of silver reducedduring development. This result could never be obtained by following theknown processes.

The action obtained with a solution of difl'erent developers (pyrogallicacid and difierent dihydroxybenzenes such as hydroquinone,pyrocatechine, chlorhydroquinone etc.) upon exposed silver halidecolloid emulsions is not at all of the same nature as that obtained withsolutions of other chemicals (particularly alkalis) which solutions havefor the remainder a suitable composition, the latter showing very markeddifi'erences in action which manifestthemselves for instance in that apyrogallic acid developer also readily antagonistic properties of thesedevelopers is obtained by the combined developer according to theinvention.

Moreover the described combination has proved to be particularlysuitable in those cases in which the penetration of the active lightwith exposure, and of the hardening developer into the emulsion with thedevelopment, ought to occur from different sides, for instance withfilms exposed from the back.

The technical conditions are the following: The developer penetratinginto the layer first comes into contact with substantially non-exposedemulsion. and only by further penetration comes upon exposed portions ina-progressive manner. hardened image is obtained with a purepyrocatechine alkali developer (as for instance proposed by Koppmann inGerman Patent 309,193) than with the pyrogallic acid-pyrocatechinemixture according to the invention. Since with a pure pyrocatechinedeveloper (according to German Patent 309,193) a hardening occurs at allexposed portions of the image, which hardening resists washing with warmwater,'this hardening viewed from the exposed side, penetrates very farinto the layer. Often it will be found that the hardening on theopposite side from the side of exposure of the layer is more intensivethan at the side of exposure proper, although naturally the largerquantity of light must have acted upon the side of exposure. This seemsto be due to the fact, that simultaneously with the hardening of thegelatine its perviousness to the pure pyrocatechine developer isreduced. In some cases lighter portions of the image is washed away. Nowfor good results the compensation of the Therefore, an essentiallydifferent this reduction may proceed so-far as to prevent the hardening,even with development for a prolonged period, to penetrate into theother side of the layer. This causes the image during the subsequenttreatment with warm water to be entirely or partially washed away, or itcauses the formation of an insoluble film which entirely or partiallygrows together with the developed hardened relief.

This phenomenon does not present itself with the mixture of adihydroxybenzene and mmdroxybenzene according to the invention. Withsuch a mixture. on the contrary, the highest degree of hardening of allportions of the image is present at the point acted upon by the largestquantity of light and consequently at the side of exposure. It may beaccepted that this hardening in accordance with the reacting quantity oflight is due to a better penetrating ability of the developing mixtureof the new composition.

With the same composition of the developer the developing and hardeningcapacity of the pyrogallic acid is considerably higher than that of anydihydroxybenzene. It consequently appears that, as a'rule, theproportion of trihydroxybenzene in the developer according to theinvention ought to be smaller than the proportion of thedihydroxybenzene namely in the ratio of 1:5 or 1:30. With a suitablepyro allic acid pyrocatechine developer the ratio of py o allic acid topyrocatechine is for instance 1:15. If the relative percentage of pyroallic acid is increased the action proceeds farther intothe layer butalso the solubility of the non-exposed portions of the layer decreases.If the relative percentage of pyrogallic acid is reduced the propertiesof the pyrocatechine dominate. Consequently the properties of thedeveloping mixture may be controlled at will in connection with thepurpose for which the hardened image law be employed. A furtherimportant improvement is obtained by considerably reducing the usualaddition of alkali which highly promotes oxidation. The usual mixtureapplied for instancewith Koppmann's process (so called Jos-Pe-process)comprises about ten times more parts by weight of potash than ofpyrocatechine. 0n the other hand 'for instance developers arerecommended with a ratio of 1 part by weight of pyrocatechine to 7.5parts by weight of potash. Other proposals go in the same direction. Inany case the proportion of the alkali carbonates contained in hardeningdevelopers has always been taken considerably larger than the proportionof the usually employed pyrocatechine or other compounds which act .in alike manner. However, with all these formulas it appears that thedeveloping mixture which is to be prepared immediately before use ishighly oxidized during the normal developing period. In addition theretothe danger of insolubillty arises, which is obviated by the inven-'tion, according to which the proportion of potash to be added forinstance to the above described pyrogallic acid-pyrocatechine developeror to the usual pyrocatechine developer is at the utmost equal to theproportion of pyrocatechine and preferably in the ratio of 1:2 to 1:2.5;It is evident that instead of potash other alkalis may be used, in whichevent the proportion thereof is to be calculated in accordance withtheir alkalinity with respect to the proportion of potash mentioned asunit.

For instance 0.84 part by weight of sodium carbonate corresponds to 1part by weight of potash etc. Likewise it'is possible to replace thepyro- 'catechlne by other suitable developers.

A developing mixture with reduced percentage of alkali contains forinstance about 2.5 parts by weight of pyrocatechine, 1 part by weight ofpotash and diiferent proportions of water depending upon the purpose forwhich'the developer is to be used, for instance 150 parts by weight ofwater and if desired an addition of pyrogallic acid. With this developerfaultless hardened images can be obtained for instance for use inetching for intaglio printing and the developing mixture which is to beprepared directly before use faultlessly prevents the layer being washedaway insoluble.

With the application of the described process it is not imperativelynecessary to mix the stock solutions of the developer and the alkali; itis rather advantageous to apply in the sense of the inventive idea adeveloping process in which the layer to be developed is first treatedin a known way with a bath of one solution until it is soaked therewithand then with the other solution, if desired with a third solution inwhich the development is completed. As a normal layer of emulsion canabsorb only restricted proportions of liquid and as a smaller proportionof alkali is used as developer the first bath is preferably an alkalinebath. Its alkali content depends upon the absorbing properties of theemulsion with respect to liquids or upon the thickness of the layer ofthe emulsion respectively.

A potash content of 25 per cent (or a corresponding ratio of otheralkalis for instance 20 per cent of caustic soda) appears to begenerally suitable. The concentration of the second bath (containing thedeveloper composed of pyrogallic acid and a dihydroxybenzene in theproportions previously given) or of asecond bath of pyrocatechine and athird bath of pyrogallic acid is of minor importance. This concentrationmay be chosen at will but with weaker concentration the developmentshould be prolonged. The described embodiment in which separate bathsare applied has the advantage that as a matter of course an oxidation ofthe developer present in the bath only can take place in the secondbath, further that only very small proportions of alkali come intocontact with a continually fresh proportion of the developer in thesecond bath and that the contact'between alkali and developing substanceis most lasting in the depth of the 2 layer to be developed. Thisprocess causes a pronounced development in the depth. It is particularlysuitable if the latent image is present in the depth of the layer (forinstance with films exposed from the back) and if fogging andconsequently the formation of a film at the upper side of the layer isto be avoided.

In the above description the developing substance pyrocatechine has onlybeen mentioned as an example. Making allowance for the differentdeveloping capacities of the diiferent developing substances previouslymentioned, any other developing dihydroxybenzene, such as hydroquinoneor chlorhydroquinone may be used with pyrogallic acid in performing theprocess.

I claim:

1. The process for effecting a hardening development of latentphotographic images upon silver halide emulsions comprising: developingand hardening said emulsion in a solution consisting of flve to thirtyparts of a dihydroxybenzene capable of developing exposed silver andalso this layer becoming halide emulsions to one part of pyrogallic acidand an alkali accelerator in water, the ratio of the alkali to thedeveloper content being not greater than one to one by weight whenmeasured terms of potassium carbonate.

2. A process for effecting a hardening development of latentphotographic images upon silver halide emulsions comprising: treatingthe said emulsion in an alkali accelerator, then developing the latentimage in an aqueous solution consisting of live to thirty parts of adihydroxybenzene capable of developing exposed silver halide emulsionsto one part of pyrogallic acid.

3. A hardening photographic developer consisting of five to thirty partsof a; dihydroxybenzene 35 capable of developing exposed silver halideemulsions to one part of pyrogallic acid, and an alkali accelerator inwater, the ratio of the alkali to the developing substances being notgreater than one to one when measured in terms of potassium carbonate.

4. A hardening photographic developer consisting of an aqueous solutionof a dihydroxybenzene capable of developing exposed silver halideemulsions, a trihydroxybenzene and an alkali accelerator, the ratio ofthe alkali accelerator to the developing substances being not greaterthan one to one when measured in terms of potassium carbonate.

FERDINAND LEIBER.

